@Wyorp Rock is correct. Chickens are simple creatures. However the sky is the limit when it comes to us making chicken keeping complicated. It's best to keep it simple.
Common sense rules. Purchase birds from hatcheries: Dont buy or take in chickens from bird shows, Craigslist, farmer down the road, certain breeders, rescues etc...
When you go to a feed store, dont wear the same shoes when walking into an area where your chickens are located when you get back home, simply change shoes and put on your chicken shoes.
I know the temptation is there but if you MUST HAVE a bird from a breeder, quarantine it a good distance away from your existing flock for at least 4 weeks and dont go nowhere near your existing flock until you change clothes and shoes, and wash up. This will give you time to inspect the new bird(s) for external parasites and treat accordingly as well as worming them. Most poultry diseases will show symptoms in the 4 week quarantine period. If anything, the stress of quarantining will set off disease symptoms, then you can cull.
Look for anything out of the ordinary when acquiring birds from a breeder. Dont be afraid to walk away if there's a single issue. It'll save you heartache and money, not to mention playing nurse maid to a bunch of sick chickens.